1. Field of the Invention
The present invention relates to communications systems, and in particular, but not exclusively, to a communication system wherein a user is arranged to be registered and/or authenticated with the system.
2. Description of the Related Art
An exemplary IP communications network has been described in Release 5 of the specifications of the 3rd Generation Partnership Project (3GPP). Different technical specifications (available at the 3gpp.org website) address various respective aspects of the network.
3GPP Technical Specification 3G TS 24.229: “SIP Multimedia Call Control Protocol based on SIP and SDP” (TS 24.229 v2.0.0 (2002-02)), which is herein incorporated by reference in its entirety, addresses the call control protocol between a mobile device (i.e., user equipment (UE), subscriber, etc.) and various network elements such as a Serving Call State Control Function (S-CSCF), Proxy Call State Control Function (P-CSCF), and Interrogating Call State Control Function (I-CSCF). Chapter 5.4.1 of TS 24.229 addresses registration and authentication of a UE with a network element, such as a S-CSCF and indicates that if an authentication response from a UE during registration is incorrect, the network either: (1) attempts a further authentication challenge; or (2) deregisters the user and terminates any ongoing sessions for all public user identities associated with the private user identity being authenticated, and releases resources allocated to those sessions. TS 24.229 further specifies that, if the authentication response from the UE is incorrect for three consecutive attempts, then the S-CSCF deregisters the user and terminates any ongoing sessions for all public user identities associated with the private user identity being authenticated, and release resources allocated to those sessions.
A problem with this procedure is that, if a user's IP Multimedia Private Identity (IMPI) becomes known to another person, that other person (fake user) can send fake registration requests to the network which includes the user's IMPI. When the network sends an authentication challenge, the fake user will not generate correct authentication responses because the fake user does not have the necessary security (i.e., ISIM card) in the UE. Because of the incorrect authentication response, the network element may deregister the (genuine) user, dropping all of the ongoing calls of the genuine user. This oversight renders the network's subscribers susceptible to denial of service (DoS) attacks.
TS 33.203 v2.0.0 (2002-03), which is herein incorporated by reference in its entirety, addresses access security for IP-based services. TS 33.203 proposes that, even after unsuccessful re-registration, an IP Multimedia Core Network Subsystem (IMS) subscriber (i.e, mobile device) remains registered until the timer set for the next re-registration is expired. Before that, a registration flag is kept in the Home Subscriber Server (HSS) to the value registered even if the authentication was unsuccessful. The S-CSCF does not remove the data about a subscriber's registration and the P-CSCF shall keep the existing Security Association (SA).
The proposal in TS 33.203 is an attempted solution to the DoS problem in TS 24.229. However, this proposal is problematic in that even though a fake subscriber sends an invalid response to the network and the network loses its trust in the subscriber, the subscriber is still registered up until the time that its expiration timer expires.